(1) Field of the Invention
The present invention relates to a non-chromate metal surface treating composition for increasing the adhesion of a metal's surface to any one of a group of layers applied thereto, such as corrosion resistant layers, and method of applying same. More particularly, the present invention relates to a metal surface etching solution wherein a chromate, such as sodium dichromate dehydrate, or an oxide of chromium, such as chromium trioxide, is replaced with a titanate, namely sodium metatitanate or an oxide of titanium, namely, titanium dioxide, respectively.
(2) Description of the Prior Art
It is known that solutions containing hexavalent chromium can be used to treat the surface of a metal as etching agents to increase the adhesion of layers which are subsequently applied thereto, such as protective coatings. However, although hexavalent chromium-containing solutions are efficient etching agents, they are also highly toxic and adversely affect the environment and human health. For this reason, many chromate-free solutions for treating metal surfaces have been proposed.
Thus, various non-chromate metal surface treatments, such as disclosed in Tomlinson U.S. Pat. No. 5,759,244, the disclosure of which is incorporated by reference herein, have been disclosed which can increase the adhesion of a metal's surface to a layer subsequently applied thereto. Many of these metal treatments are based on group IV-B metals such as titanium, zirconium and hafnium. For example, U.S. Pat. No. 5,868,872 to Karmaschek et al discloses a chromium-free aqueous bath solution for non-rinse treatment of aluminum and its alloys. The solution comprises zirconium and titanium, orthophosphate, fluoride and a water-soluble or homogeneously water-dispersible organic film former. When applied, the solution is dried on the surface of the aluminum without rinsing. Similarly, U.S. Pat. No. 5,897,716 to Reghi et al discloses a chemically and thermally stable chromate-free aqueous liquid treatment for metals which increases the adhesion of protective layers to the metals' surfaces. The chromate-free aqueous liquid comprises components selected from the group consisting of H2TiF6, H2ZrF6, H2HfF6, H2SiF6, H2GeF6, H2SnF6, HBF4, and mixtures thereof.
The shortcoming of conventional non-chromate metal surface treatments, such as those described above, is that they cannot be integrated into and employed in place of chromium-containing compounds in current metal treatment solutions which otherwise would contain chromium. As such, conventional non-chromate metal surface treatments are usually so different from previously employed chromate-containing metal surface treatments that significant changes are required to be made in the metal treating process and in the production of the metal surface treatment itself. These changes can amount substantial expenditures and usually require additional approval from Department of the Navy. Thus, there is a need for “drop-in replacements” that can be employed in place of chromium-containing compounds, such as sodium dichromate, now used in conventional chromate-containing metal treatment solutions. “Drop-in replacement” refers to a compound that can be employed in a metal surface treatment solution in lieu of a chromium-containing compound without requiring any or substantial changes in the make-up of the metal surface treatment process or metal surface treatment solution.